1) In A New Hope on the Death Star, the reason they sucked at aiming was just to chase the Rebels off the station and lead them to their base on Yavin 4(They planted a beacon on the Millennium Falcon)

2) The helmet's eye-holes may interfere with their aim. The helmet has targeting systems but the eye-holes are too small to see through!(Jeez I hope they fixed this problem by introducing the Storm Trooper Phase 2 helmets!)